1. Field of the Invention
The present invention relates in general to electromagnetic copy mediums and the method for producing positive and negative copies electrostatically and more specifically to such mediums that are formed of an insulative top layer, an electrically conductive bottom layer, and a photoconductive layer interposed between the insulative and electrically conductive layers.
2. Description of the Prior Art
A number of patents disclose processes for producing electrostatic images by the employment of multi-layer copy mediums having an insulative layer, an electrically conductive layer, and a photoconductive intermediate layer. Such disclosures are taught in patents to Hall, U.S. Pat. No. 3,234,019, Watanabe et al. U.S. Pat. Nos. 3,457,070 and 3,536,483, and Zweig, U.S. Pat. No. 3,722, 992. These patents also teach the transfer of an image produced on the surface of the photoconductive layer to the surface of the insulative layer in order that the image is preserved irrespective of an exposure to light. Image transfers to the surface of the insulative layer are also advantageous to avoid the dissipation of the image forming voltages through conduction across the photoconductive layer.
It is also known in the art to employ a crystalline photoconductive-pyroelectric compound together with an electrically conductive layer to form a copy medium as disclosed in patent to Kiess, U.S. Pat. No. 3,713,822. To produce a copy by the use of such a medium in accordance with the teachings of the Kiess patent, first the photoconductive-pyroelectric compound is heated in the dark to develop a positive electrostatic charge on one surface of the compound and a negative electrostatic charge on the opposite surface. Thereupon, the charged compound is exposed to a light image, which converts the exposed image areas from a low conductivity to a high conductivity, permitting the negative and positive charges of the exposed areas to combine. This results in a reduction of surface charge in the exposed areas to produce an electrostatic latent image charge pattern on the photoconductivepyroelectric compound. However, when a charge representative of the image is formed on the photoconductive layer, it musst be immediately developed to avoid dispersion of charge as the result of an unintentional exposure of the photoconductive layer to light or a breakdown of that layer.
The present invention provides for an improved copy medium and process using the medium for producing an image on the surface of an insulative layer by means of an image transfer from the surface of a photoconductor, with such process also being usable with other prior art copy mediums.